GUS instrument mappings bug

The GUS instrument mappings bug is a bug in vanilla Doom related to how music is played back when using a Gravis Ultrasound card.

The bug relates to how the DMXGUS configuration lump is parsed by Doom's music code; MIDI instruments are mistakenly mapped to the wrong instruments. Since the sound cards had limited memory in which to load the sound banks, some instruments can be substituted with others. The bug comes from a confusion in the parser, so instead of remapping an instrument to its substitution, it remaps them to the first instrument that shares the same substitution.

A prominent example of this can be heard in "Running from Evil", the theme for MAP01: Entryway (Doom II) where a dissonant clanging sound replaces a percussion drum instrument that is supposed to be played. The DMXGUS mapping substitutes steel drums (General MIDI instrument 114) with melodic toms (GM instrument 117), but because of the bug, the inverse happens and steel drums are played where the song calls for melodic toms.

The bug can be worked around by loading a modified DMXGUS lump.